I always had the impression that USB sound cards were really usable on a DENEB because of the ZorroIII interface. Clockport's bandwidth is much much slower than ZIII.
Is there any chance that the stutter you experience is because of the limitations of the clockport's bandwidth?
Because if that't the case I can hardly see any way to overcome this.

Bandwidth is not really an issue for audio. We're talking under 200k/second, which is perfectly possible with the clockport and of course with Z2/Z3 interfaces. The trick is to start a number of USB transfers at the same time: With RapidRoad, the driver can start up to 8 realtime-ISO transfers at the same time (to my knowledge, all previous solutions used a maximum of two simultaneous transfers). Poseidon had to be updated in order to support this number of RT-ISO transfers. I'll upload this update to my Wiki sometime this weekend.

Also, don't get me wrong: We're not experiencing stutter on the clockport. It works fine on the clockport at the programmer's site, using a B1260. It has not been tested on the X-Surf-100 yet. That's only because I don't have a USB sound card - yet. It should arrive today.

Still, on slower Amigas, the 7ms minimum time between data chunks to be moved to/from the sound card may be a dealbreaker. If a SCSI DMA transfer is running during that time, Poseidon *may* pick up data too late (or send it too late), resulting in stuttering audio. Note that this is merely a possibility, not an observation. I'm trying to tell everyone that this is not yet tested.